The use of reinforced concrete for structural elements of buildings is well known. The loads in concrete structures are transferred by both the concrete and the reinforcement within the structural elements. For many building applications, formwork for the concrete is assembled in situ, reinforcement is placed (typically in the form of steel rebar, often pre-stressed), and the concrete is poured and allowed to cure.
Alternately, precast concrete building elements are sometimes used, with the reinforcement already located therein and often extending out from one or more surfaces thereof. The use of precast elements can offer several advantages, including simplifying and speeding construction, reducing susceptibility to weather and environmental conditions on site, affording greater consistency and quality control for precast elements made in a more controlled, factory setting, etc. Examples of the advantageous use of precast concrete structural elements can be found in the inventor's prior U.S. Pat. No. 4,505,087, the contents of which are herein incorporated by reference in their entirety.
A conventional downside of the precast building elements is the complexity that can be involved when structurally tying one such element into other structural elements. Often, the reinforcement from adjacent elements are bolted or welded together, which can be very time consuming, require highly skilled laborers and special equipment. Sometimes the connection problem is largely avoided in low-rise structures by al the precast elements tall enough for the full height of the structure.
Referring to FIG. 9, a lap splice 170 can be used to tie adjacent elements together, such as upper and lower precast columns 114, 124. The lap splice does not require reinforcement to be bolted or welded together, but simply to overlap for a predetermined splice length 176 in the space between the precast elements, which space is subsequently filled with concrete that cures around the splice 170 and holds the precast elements 114, 124 together.
While this appears, on the face of it, to be a rather simple solution, the problem is that engineering concerns and code requirements will often require a splice length 176 so considerable that it becomes impractical to accommodate the space needed between the precast elements 114, 124. Although the particular equations used to calculate the required splice length can be rather complex, a good approximation for illustrative purposes is that the splice length must be 40 times the diameter of reinforcement for splices in tension and 30 times the diameter of reinforcement for splices in compression.